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C Pérez-García

Publications and source records attributed to C Pérez-García.

At least 19 recordsLinked to original sources
PubMed2003

Transverse instabilities in chemical Turing patterns of stripes.

We present a theoretical and experimental study of the sideband instabilities in Turing patterns of stripes. We compare numerical computations of the Brusselator model with experiments in a chlorine dioxide-iodine-malonic acid (CDIMA) reaction in a thin gel layer reactor in contact with a continuously refreshed reservoir of reagents. Spontaneously evolving Turing structures in both systems typically exhibit many defects that break the symmetry of the pattern. Therefore, the study of sideband instabilities requires a method of forcing perfect, spatially periodic Turing patterns with the desired wave number. This is easily achieved in numerical simulations. In experiments, the photosensitivity of the CDIMA reaction permits control and modulation of Turing structures by periodic spatial illumination with a wave number outside the stability region. When a too big wave number is imposed on the pattern, the Eckhaus instability may arise, while for too small wave numbers an instability sets in forming zigzags. By means of the amplitude equation formalism we show that, close to the hexagon-stripe transitions, these sideband instabilities may be preceded by an amplitude instability that grows transient spots locally before reconnecting with stripes. This prediction is tested in both the reaction-diffusion model and the experiment.

Journal Article↗
PubMed2003

Convective instabilities in two superposed horizontal liquid layers heated laterally.

This work is devoted to the theoretical study of the stability of two superposed horizontal liquid layers bounded by two solid planes and subjected to a horizontal temperature gradient. The liquids are supposed to be immiscible with a nondeformable interface. The forces acting on the system are buoyancy and interfacial tension. Four different flow patterns and temperature profiles are found for the basic state. A linear perturbative analysis with respect to two- and three-dimensional perturbations reveals the existence of three kinds of patterns. Depending on the relative height of both liquids several situations are predicted: either wave propagation from cold to the hot regions, or waves propagating in the opposite direction or still stationary longitudinal rolls. The behavior of three different pairs of liquids which have been used in experiments on bilayers under vertical gradient by other authors have been examined. The instability mechanisms are discussed and a qualitative interpretation of the different behaviors exhibited by the system is provided. In some configurations it is possible to find a codimension-two point created by the interaction of two Hopf modes with different frequencies and wave numbers. These results suggest to consider two liquid layers as an interesting prototype for the study of propagation and interaction of waves in the context of the Bénard-Marangoni problem.

Journal Article↗
PubMed2003

Histamine H3 receptor: a potential drug target for the treatment of central nervous system disorders.

Histamine H(3) receptors were first described in the eighties but finally cloned four years ago. They are G-protein coupled, mostly presynaptic, and are involved in the control of the synthesis and/or release of different neurotransmitters both in the central nervous system and the periphery. The availabiliy of specific ligands has permitted the study of potential therapeutic applications of either stimulating or blocking the function of these receptors. There is experimental evidence that drugs targeted at histamine H(3) receptors could be beneficial for neurodegenerative diseases such as Alzheimer and Parkinson's disease, epilepsy, drug abuse and several affective, appetite and sleeping disorders, among others. This review presents recent advances in this field.

Animals↗
PubMed2001

Stability of Turing patterns in the Brusselator model.

The selection and competition of Turing patterns in the Brusselator model are reviewed. The stability of stripes and hexagons towards spatial perturbations is studied using the amplitude equation formalism. For hexagonal patterns these equations include both linear and nonpotential spatial terms enabling distorted solutions. The latter modify substantially the stability diagrams and select patterns with wave numbers quite different from the critical value. The analytical results from the amplitude formalism agree with direct simulations of the model. Moreover, we show that slightly squeezed hexagons are locally stable in a full range of distortion angles. The stability regions resulting from the phase equation are similar to those obtained numerically by other authors and to those observed in experiments.

Journal Article↗
PubMed2001

Stability of hexagonal patterns in Bénard-Marangoni convection.

Hexagonal patterns in Bénard-Marangoni (BM) convection are studied within the framework of amplitude equations. Near threshold they can be described with Ginzburg-Landau equations that include spatial quadratic terms. The planform selection problem between hexagons and rolls is investigated by explicitly calculating the coefficients of the Ginzburg-Landau equations in terms of the parameters of the fluid. The results are compared with previous studies and with recent experiments. In particular, steady hexagons that arise near onset can become unstable as a result of long-wave instabilities. Within weakly nonlinear theory, a two-dimensional phase equation for long-wave perturbations is derived. This equation allows us to find stability regions for hexagon patterns in BM convection.

Journal Article↗
PubMed1999

Effects of histamine H3 receptor ligands in experimental models of anxiety and depression.

Histamine H3 receptor ligands have been proposed to be of potential therapeutic interest for the treatment of different central nervous system disorders; however, the psychopharmacological properties of these drugs have not been studied extensively. In this work, we investigated the possible involvement of histamine H3 receptor function in experimental models of anxiety (elevated plus-maze) and depression (forced swimming test). Male Sprague-Dawley rats were treated i.p. with the histamine H3 receptor agonist R-alpha-methylhistamine (10 mg/kg) or the histamine H3 receptor antagonist thioperamide (0.2, 2 and 10 mg/kg) and 30 min afterwards the time spent in the open arms of an elevated plus-maze was registered for 5 min. The immobility time of male OF1 mice in the forced swimming test was recorded for 6 min, 1 h after the i.p. administration of R-alpha-methylhistamine (10 and 20 mg/kg), thioperamide (0.2, 2, 10 and 20 mg/kg) or another histamine H3 receptor antagonist, clobenpropit (5 mg/kg). The locomotor activity of mice was checked in parallel by means of an activity meter. Both saline controls and active drug controls were used in all the paradigms. Neither thioperamide nor R-alpha-methylhistamine significantly changed animal behaviour in the elevated plus-maze. R-alpha-methylhistamine and the higher dose of thioperamide assayed (20 mg/kg) were also inactive in the forced swimming test. By contrast, thioperamide (0.2-10 mg/kg) dose-dependently decreased immobility, the effect being significant at 10 mg/kg (33% reduction of immobility); clobenpropit produced an effect qualitatively similar (24% reduction of immobility). None of these histamine H3 receptor antagonists affected locomotor activity. These preliminary results suggest that the histamine H3 receptor blockade could be devoid of anxiolytic potential but have antidepressant effects. Besides, the stimulation of these receptors does not seem to be followed by changes in the behavioural parameters studied.

Analysis of Variance↗
PubMed1998

Histamine H3 receptor desensitization in the guinea-pig ileum.

Histamine H3 receptor ligands are usually tested in guinea-pig intestine preparations. A possible desensitization of agonist-induced twitch inhibition was studied in longitudinal muscle-myenteric plexus from ileal segments. A cumulative concentration-response curve for R-alpha-methylhistamine was made; when a second curve was made 30 min afterwards, a marked decrease of pD2 and a more modest decrease of Emax were observed without changes in tissue sensitivity to electrical stimulation or morphine inhibition. At 120 min, pD2 and Emax were not different from those for the first curve. Receptor desensitization seems homologous and reversible and could interfere with repetitive testing of histamine H3 receptor ligands.

Animals↗
PubMed1993

[Still's disease: atypical manifestations and potential markers of severe visceral disease].

We present two patients with Still's disease which, in addition to the typical clinical manifestations, showed rare visceral alterations. The first case referred abdominal pain and a sustained hyperamilasemia was detected. The second case suffered a severe multiple organic failure, characterized by hepatic failure, with electric signs of encephalopathy, hydroelectrolytic disorder, rhabdomyolysis and disseminated intravascular coagulation. During this episode, we detected hyponatremia, reduction of leukocytes and normalization of GSR, modifications already described by other authors in cases similar to these ones, although without considered relevant until now. We comment these results and their value as markers of visceral affectation.

Adolescent↗